In liquid crystal display units mainly employed in these days, use is made of the so-called TN mode wherein liquid crystal molecules having a positive dielectric anisotropy are aligned in a twisted state by about 90°. A liquid crystal display unit of the TN mode comprises substrates having a pair of transparent electrodes, a liquid crystal cell comprising nematic liquid crystals enclosed between the substrates and polarizing plates provided in both sides thereof. Compared with LCDs of other modes, liquid crystal display units of the TN mode are advantageous in showing a relatively high response speed of several ten milliseconds and achieving a high display contrast. However, they suffer from some problems due to viewing angle characteristics such that the display color and display contrast vary depending on viewing direction and, therefore, are still inferior in display performance to CRT.
In a liquid crystal display unit of the TN mode, display is made by applying an electric filed perpendicularly to the substrates. By switching on/off the electric field application, the major axis of liquid crystal molecules are aligned almost perpendicular with the substrates/almost parallel therewith. Thus, display is made owing to the change in the major axis direction of liquid crystal molecules on the face perpendicular to the substrates. Accordingly, the retardation of the liquid crystal layer largely varies depending on the viewing direction and, as a result, the display color and display contrast widely vary. To improve these viewing angle characteristics, it is proposed to provide an optical compensation layer between a polarizer and a TN mode liquid crystal cell and there have been proposed various optical compensation sheets. Although the viewing angle can be enlarged to a certain extent by providing such an optical compensation sheet, no such a wide viewing angle as being available as an alternative to CRT can be established in practice so far.
There has been also proposed a display system wherein an electric filed is applied in the direction not perpendicular to substrates as in the above cases but almost parallel therewith (JP-A-7-261152 and ASIA DISPLAY'95 (published by The Institute of Television Engineers of Japan & The Society for Information Display, pp. 577 to 580 and 707 to 710)). This display system wherein an electric field is applied almost in parallel to substrates is called the IPS (In-Plane Switching) mode. In a liquid crystal display unit of the IPS mode, liquid crystal molecules have homogeneous orientation almost parallel to the substrate face in the nondriven state. Therefore, light penetrates through the liquid crystal layer while little changing the polarizing face. Therefore, almost complete black display can be made in the nondriven state by providing polarizing plates above and below the substrate.
In the IPS mode, almost complete black display can be made in the normal panel line direction. In the case of observing the panel from a direction deviating from the normal line, however, there arises a problem that the viewing angle is narrowed because of light leakage that is unavoidable depending on the characteristics of the polarizing plates in the direction deviating from the light axis of the polarizing plates provided above and below the liquid crystal cell, and unignorable phase contrast caused by the homogeneously oriented liquid crystals and the transparent protective films.
To overcome this problem, use is made of a polarizing plate in which the geometric axis deviation in the polarizing plate occurring in looking from an angle is compensated by an optical compensation sheet. Polarizing plates exerting this effect are disclosed in, for example, JP-A-4-305602 and JP-A-4-371903. However, a sufficiently wide view angle can be hardly obtained in practice by using known optical compensation sheets.
In the polarizing plate reported by JP-A-4-305602, an optical compensation sheet is employed as a protective film for a polarizer. Although this polarizing plate affords favorable viewing angle characteristics in the common use environment, the polarizer suffers from a dimensional change at a high temperature or a high humidity and, in its turn, the protective film directly laminated thereon is deformed too. As a result, there arises a problem that the phase contrast of the optical compensation sheet employed as the protective film departs from the desired range so that the effects thereof cannot be stably maintained.
In JP-A-4-371903, an optical compensation sheet is laminated on a polarizing plate with the use of a triacetyl cellulose film (TAC film) having been commonly employed as a protective film. Since no stress is directly loaded on the optical compensation sheet in this case, the phase contrast of the optical compensation sheet remains stable. However, the TAC film has an unignorable phase contrast, which makes it difficult to design the optical compensation layer for compensating. Similar to the above-described case, moreover, the TAC film is stretched due to a dimensional change of the polarizer at a high temperature or a high humidity so as to cause a change in the phase contrast, which makes it impossible to establish the desired object.
In JP-A-2004-4641 and JP-A-2004-4642, a film containing a norbornene resin and a film containing a resin having imido group together with a resin having phenyl group and nitrile group are laminated as protective films. Since the phase contrast can be lowered in these cases, an optical compensation layer can be easily designed. However, these synthetic resins generally have hydrophobic nature, which frequently results in a problem in adhesion to a polarizer. That is, these resins are liable to peel off. In these cases, moreover, moisture transmission from the polarizer can be scarcely expected and thus moisture remains inside, which brings about a problem of worsening in the polarizing plate performance. Similar to the above-described case, furthermore, the problem of change in the phase contrast is more serious in such a synthetic resin film than in the TAC film, which makes it impossible to establish the desired effect.